The Chemical Elements Standard Atomic Masses

Element Symbol Atomic Number Atomic Mass 

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Named by a French chemist, Lavoisier, hydrogen (H) is the first chemical element of the periodic table of elements with an atomic number of one. At standard temperature and pressure, hydrogen is a colourless, tasteless, odourless and easily flammable gas. With its atomic mass of 1.00797 g/mol, hydrogen is the lightest element. The British scientist, Henry Cavendish, was the first to identify H as a distinct element in 1766, publishing precise values for its specific weight and density (NHA, 2007). 

The most basic unit of a chemical element that can undergo chemical change is an atom. Atoms of any element are identified by the number of protons and neutrons in the nucleus. The number of protons in the nucleus of an element is given by the atomic number. Hydrogen has one proton in its nucleus so its atomic number is one. The atomic number of carbon is six, because each carbon atom contains six protons in its nucleus. Besides protons, the nucleus contains neutrons. The number of protons plus the number of neutrons is the mass number of an element. A standard method of symbolizing an element is to write the elements with the mass number written as a superscript and the atomic number as a subscript. Carbon-12 would be written as... 

It should be stressed that u, as the standard for comparing the masses (weights) of all chemical elements in all kinds of chemical compounds, refers not simply to tlie carbon atom, but rather to one very specific isotope, carbon-12. There are two stable isotopes of carbon, 12C and 13C, and four known radioactive isotopes, 1(,C, 1C, 14C, and 15 C. 

In Table 8.18, mx, is the exact mass of the isotope of X that differs by i integer mass units from m, and is the corresponding relative abundance. Isotopes of relative abundance lower than 0.001 are neglected. Suchcases are with mn i = 2.014102 Da, j = 0.00015 and with = 35.967079 Da, and = 0.00020. A more extensive list of exact masses and isotope distribution for most elements is available online, e.g. [288], as well as in many standard reference books. The exact isotope masses are weighted by their intensities to calculate the average atom mass of a chemical element ... 

Atoms of different elements have different masses. When we perform chemical calculations, we need to know how heavy one atom is compared with another. The mass of a single atom is so small that it is impossible to weigh it directly. To overcome this problem, we have to weigh a lot of atoms. We then compare this mass with the mass of the same number of standard atoms. Scientists have chosen to use the isotope carbon-12 as the standard. This has been given a mass of exactly 12 units. The mass of other atoms is found by comparing their mass with the mass of carbon-12 atoms. This is called the relative atomic mass, A. ... 

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